1. Field of the Invention
The invention is concerned with electrically pumped vertical cavity laser structures designed with a view to low lasing threshold current. While discrete devices are contemplated, an important aspect is in terms of integratable structures now made practical because of low threshold (and consequently, lowered heating).
2. Description of the Prior Art
A number of recent developments have directed attention to the need for inexpensive reliable lasers--both discrete and integrated. As important as the laser is in so many specialties--optical communications, medical diagnosis, weaponry, basic scientific inquiry, to name a few--development has lagged in very meaningful terms. The void resulting from unavailability of high density, small dimension, integrated devices is particularly noteworthy.
Advantages that would flow from introduction of a truly integratable laser e.g. permitting element-to-element communication on a chip, chip-to-chip communication, and performance of switching, amplification, and other functions now dependent on semiconductor elements are important and varied. It is estimated that computation speed could be doubled by use of all-optic circuitry. Projection displays and laser printers utilizing high density laser arrays would profoundly affect the industry.
Optical interconnect is generally contemplated in terms of inclusion in integrated circuits constructed of material suitable for performing electronic as well as optical functions. Use of e.g. III-Vs and other compound semiconductors of appropriate bandgap values will likely serve in Opto Electronic Integrated Circuits. Optical functions may play an important role in conjunction with silicon technology as well. Silicon circuitry likely to continue its electronic dominance for some time may be provided with optical capability by superimposition of chips of other material; for example, to provide for chip-to-chip communication.
Rapid introduction of fiber optics has already revolutionized long distance communications. Availability of inexpensive lasers will extend fiber optics both to the loop plant and to terminal connection (both in industry and the home).
Other implications include steerable beams for pointing, and high power arrays.
Such potential uses have not been overlooked. Intense worldwide effort has resulted in very significant advances. Recent work has produced devices of small size benefiting from sophisticated fabrication techniques such as Molecular Beam Epitaxy and Metal Organic Chemical Vapor Deposition. MBE, in particular, must be credited for emergence of layered structures requiring dimensional control, dimensional homogenity, freedom from defects and surface smoothness, all in terms of a fraction of a wavelength (of contemplated radiation). Surface emitting vertical lasers may utilize such layered structures in the active region (e.g. in the form of quantum wells), as well as for defining the cavity (in the form of Distributed Bragg Reflectors.
A major obstacle to more general implementation of the laser is temperature increase due to heat generation attending lasing. This is particularly true for integration in which the problem is only aggravated for the higher packing densities which is a major thrust for integration. For any given level of efficiency of operation, need for heat dissipation decreases as lasing threshold decreases. Lasing threshold is in turn dependent--generally linearly dependent--on the volume of active material. Major effort has been in the direction of decreasing this volume.
The cavity structure is an important determinant for the thickness of active material between the mirrors. The ultimate requirement for any lasing structure is for a reflectivity/loss characteristic which accommodates per-pass gain for the stimulated radiation. Recently announced work appears to have carried this to the ultimate limit in permitting lasing with but a single quantum well in a surface emitting structure. ("Applied Physics Letters", vol. 55, no. 24, pp. 2473-2475 (1989). The work depends upon use of DBR mirrors of near-perfect reflectivity (reflectivity of 99.9% for a laser wavelength of 980 nm resulted from use of DBR mirrors each of approximately 20 periods). While the first experiments were based on optically pumping, the authors immediately realized implications in terms of electrically pumping.
Electrical pumping, the form most commercially significant devices will take, introduces problems. The DBR, clearly the best approach so far, does not intrinsically have the desirable electrical characteristics for serving in the series electrical pump path. Optimized Bragg structures of appropriate transparency, refractive index (n) and layer-to-layer index difference (.DELTA.n) introduces a level of I.sup.2 R loss and heating to result in need for increased volume of active material (and still further heating). Beyond some level, heating is too great to be compensated by further volume increase. Modification of the DBRs to create the needed pn junction requires doping which implicitly results in scattering centers, and, therefore, in further loss (requiring further increase in volume of active material). The critical consideration in many design terms is the thickness dimension in the emission direction since, aside from diffraction effects, both loss and gain are generally linearly dependent on lateral dimensions--usually on lateral area.
One line of endeavor represented, for example, by an article in "Electronics Letters," vol. 24, no. 15, pp. 928, 929 (Jul. 21, 1988) seeks to avoid the problem by providing distinct electrical and optical paths. In the cited article, a Surface Emitting Laser having an active region of 3 .mu.m thickness and 30 .mu.m diameter was operated room temperature, pulsed at a threshold current, I.sub.th =200 mA, equivalent to a current density of 30 kA/cm.sup.2. The cavity in this instance used but a single DBR and depended on a conventional single-surface reflector on the other side. The authors' determination to put the DBR on the n-type side was no doubt dictated by the greater electrical resistance introduced by p-type doping. Other approaches also dependent upon use of distinct current and optical paths have used various means for injecting pumping currents laterally, e.g. at active material--mirror interfaces. "Applied Physics Letters", vol. 51, no. 21, pp. 1655-57 (Nov. 23, 1987).
It may be concluded that absence of truly low threshold current devices, for example, as required for ICs, is due, in one instance, to implicitly high resistance (for devices using coincident current and optical paths) and in the other, to fabrication difficulty as well as device function problems (for devices using distinct current and optical paths).